1. Field of the Invention
This invention relates to a thermal printer and, more particularly, to a feeding mechanism for a thermal printer in which there is conservation of a thermal transfer medium from which a marking material is transferred when it is softened by heat to a flowable state.
2. Description of the Prior Art
It has previously been suggested in the aforesaid Applegate et al application to have conservation of a thermal transfer medium such as an inked ribbon, for example, by underfeeding the ribbon relative to movement of a thermal printhead of a thermal printer. This results in the ribbon sliding relative to both the printhead and a recording or receiving medium such as a sheet of paper, for example, during formation of the print.
In the preferred embodiment of the aforesaid Applegate et al application, feeding of both a carrier which carries the printhead and the ribbon occurs from a single power source through using gears and selectively changing the gear ratios to feed the ribbon at different velocities relative to the velocity of the printhead. The aforesaid Applegate et al application also suggests that two separate power sources could be utilized for driving the carrier which carries the printhead and the ribbon.
The feeding of the ribbon in the aforesaid Applegate et al application is accomplished by having a pair of pinch rolls engage the ribbon after using the ribbon to print on the sheet of paper. The ribbon feeding mechanism of the aforesaid Applegate et al application also has drag brake means between the source of the ribbon and the printhead to create a tension on the ribbon at the print point.
The present invention is an improvement of the thermal printer of the aforesaid Applegate et al application. It has been found that the sliding force of the ribbon against the sheet of paper has a sufficient magnitude to pull the ribbon from the supply spool during printing in the thermal printer of the aforesaid Applegate et al application. Thus, while the ribbon feed mechanism of the aforesaid Applegate et al application conserves ribbon, accurate metering of the ribbon relative to the paper is not always obtainable because of the inability of a brake/tensioning mechanism of the ribbon feed mechanism of the aforesaid Applegate et al application to restrain the ribbon with any repeatability and reliability.
To obtain accurate metering of the ribbon relative to the sheet of paper, the ribbon feeding mechanism must pull the ribbon from its cartridge and meter it onto the sheet of paper at a consistent velocity irrespective of variations in the unwind tension and the frictional characteristics of the ribbon. This must occur without damaging the ribbon such as by scratching, wrinkling, or overstressing the ribbon, for example, prior to printing.
While the thermal printer of the aforesaid Applegate et al application conserves ribbon, there is no recognition that consistent conservation of the ribbon can be obtained where the premetering tension, which is the tension on the ribbon prior to the metering roll and is the sum of the tension created by drag brake means and the unwind tension, is maintained substantially constant and the take-up tension is maintained substantially constant as the feed mechanism of the present invention can accomplish. By maintaining the ratio of the take-up tension to the premetering tension at a selected value since the two tensions can be maintained substantially constant, slippage of the ribbon during feeding is avoided.
In the aforesaid Applegate et al application, the magnitude of the drag brake tension, which constitutes part of the premetering tension in addition to the unwind tension, is the product of the coefficient of friction between the ribbon and the pad of the drag brake and the normal force of the ribbon on the pad. The coefficient of friction does not remain constant because each ribbon has variations from other ribbons to create slight differences in the coefficient of friction with the pad of the drag brake. Further changes in the coefficient of friction occur due to wear of the surfaces over which the ribbon passes and the build up of contaminants over such surfaces. Thus, these variations in the coefficient of friction do not enable a consistent velocity to be applied to the ribbon so that the most effective conservation of the ribbon is not obtained.
It should be understood that the drag brake is opened when the drag brake tension increases beyond a predetermined tension so that there is no control of the premetering tension during this time. After a period of time, the drag brake again closes to again enable control of the premetering tension. Accordingly, an increase in the coefficient of friction causes more frequent opening of the drag brake to cause more frequent fluctuation in the premetering tension.
There also are changes in the coefficient of friction due to ribbon feed leaders, whch have a very low coefficient of friction, and the ribbon ends, which may be abrasive so as to have a relatively high coefficient of friction. Each of these has an effect on the velocity of the ribbon when these portions of the ribbon pass the drag brake.
The present invention solves the problem of the changes in the frictional characteristics of the ribbon causing variations in the premetering tension through utilizing an arrangement in which changes in the coefficient of friction between the ribbon and the material of the drag brake has no substantial effect on the premetering tension. This is accomplished through relieving the normal force of the ribbon on the pad of the drag brake as the coefficient of friction increases. Thus, there is not a linear increase in the premetering tension due to an increase in either the coefficient of friction or the normal force as occurs in the aforesaid Applegate et al application.
It also has been discovered that another effect on the feed velocity of the ribbon occurs during printing because of changes in friction between the thermal printhead and the ribbon and between the sheet of paper and the ribbon. During printing, heat from the electrodes of the printhead increases the friction between the printhead and the ribbon and decreases the friction between the sheet of paper and the ribbon. As these changes in friction occur, the ribbon has to stretch or relax depending on the net force change. The minimization of stretching or relaxing of the ribbon occurs when the stiffness of the ribbon is at a maximum in the area just prior to the printhead.
The present invention solves this problem through positioning feed means for the ribbon as close as possible to the printhead and prior to the printhead. The location of the feed means prior to the printhead eliminates the use of any type of pinch or nip rolls as is shown in the aforesaid Applegate et al application and on pages 204-207 of Volume 27, No. 1A (June 1984) issue of the IBM Technical Disclosure Bulletin.
While the aforesaid IBM Technical Disclosure Bulletin shows nip rolls engaging the ribbon prior to the printhead, these could damage the ribbon prior to printing so that the nip rolls of the aforesaid IBM Technical Disclosure Bulletin are undesirable for feeding the ribbon prior to the printhead. There also is no recognition in the aforesaid IBM Technical Disclosure Bulletin of disposing the nip rolls as close as possible to the printhead to maintain the portion of the ribbon between the nip rolls and the printhead substantially stiff to minimize stretching or relaxing of the ribbon during printing and minimize any appreciable effect on the velocity of the ribbon.
While the aforesaid IBM Technical Disclosure Bulletin discusses a constant tension on the supply spool, this is based on an assumption that the coefficient of friction between the ribbon and the braking surface pad remains constant. However, the coefficient of friction of different ribbons relative to the pad is not normally the same, and there is wear of the pad and/or build up of ink on the pad so as to change the coefficient of friction between the ribbon and the pad. As previously discussed, each of these variations has a significant effect on the coefficient of friction so as to change the premetering tension whereby it would not be constant.
To avoid damage to the ribbon prior to printing, the present invention uses a single feed or metering roll, which engages the ribbon prior to the printhead, having a high coefficient of friction with the ribbon. The ribbon also has a relatively large wrap angle around the feed or metering roll. This high coefficient of friction and the large wrap angle insures that there is no slippage of the ribbon relative to the feed or metering roll.
The ribbon feed roll of the present invention also is subjected to build up of ink on its outer surface so as to have its coefficient of friction with the ribbon changed over a period of time. The continued use of the contaminated feed roll would result in an inconsistent feed velocity because slippage of the ribbon relative to the metering roll could occur due to the reduced coefficient of friction.
This problem is solved through mounting the feed roll on a cartridge having the ribbon supply. Accordingly, the feed roll is replaced each time that the cartridge is replaced so that the need for any cleaning of the feed roll is avoided. The use of the feed roll is in accordance with the amount of ribbon employed, and the quantity of ribbon in the cartridge is selected so that there is not any significant change in the coefficient of friction between the ribbon and the feed roll before the ribbon is exhausted.
As previously mentioned, the thermal printer of the present invention has an arrangement to reduce the normal force of the ribbon on the drag brake pad as the ribbon passes through the drag brake when the coefficient of friction increases. This arrangement also is employed to guide the ribbon to cause the ribbon to wrap around more than 90.degree. of the feed roll.
The thermal printer of the present invention is an improvement of the thermal printer of the aforesaid Applegate et al application in that the ribbon may be fed at various selected velocities through its own power source. The carrier which carries the printhead is driven from a second power source at various selected velocities. While the aforesaid Applegate et al application discussed the use of two separate power sources for feeding the ribbon and driving the carrier, there is no recognition in the aforesaid Applegate et al application that more than two conservation modes (draft and quality) could be utilized although there is a description of a very large range of ratios and velocities.
The thermal printer of the present invention may operate in three different conservation modes (draft, quality, and enhanced) with the underfeed ratios (the ratio of the print speed to the ribbon feed rate) being 5:1, 2:1, and 1.2:1, respectively. With the carrier having a very high speed such as 160 characters per second with ten pitch characters, the thermal printer of the present invention is in the draft mode wherein the conservation of the ribbon enables the ribbon cost when printing in the draft mode to be in the same range as that of a fabric ribbon used with a high speed, dot matrix printer. When operating the carrier of the thermal printer of the present invention at a speed of 100 characters per second with ten pitch characters, the quality mode is obtained to produce letter quality print. At a lower speed such as 80 characters per second with ten pitch characters, the enhanced mode occurs to produce the best print.
While the thermal printer of the present invention is capable of very high speed printing when operating in the draft mode, it still does not exceed the maximum current for a selected ribbon length in a selected period of time that can be produced from the electrodes of the thermal printhead. Because of the higher speed of the printhead and the lower speed of the ribbon, there is a longer dwell at each character space by the ribbon; this allows utilization of a lower current for a selected ribbon length in a selected period of time even though the total current may be relatively large.